In the past, when driving machine tool tables and other driven bodies by a ball screw shaft and ball nut mechanism, the ball nut is attached to the driven body and the ball screw shaft is connected through a coupling etc. to the output shaft of the motor positioned at one end of the ball screw shaft. With this structure, however, it is not possible to make the apparatus compact as a whole and further it is difficult to make the force act on the center of gravity of the driven body. Therefore, a so-called hollow shaft motor has been proposed in which a ball nut is attached to the motor and a ball screw shaft is passed through the motor.
The ball screw shaft and ball nut, however, are susceptible to heat conducted from the motor, which is a heat generating body, and suffer from heat expansion, which has a detrimental effect on the ball screw shaft and ball nut mechanism, for which high precision operation is required. Further, a ball nut is attached to one end of a cylindrical shaft through which the ball screw shaft is passed. Consequently, the ball screw shaft and the cylindrical shaft form between them a bag with one end closed, so there is the problem that it is difficult to radiate away the heat.